Low contrast for the update notice on new About pages

[SergeyBiryukov]

Background: #47708, #49295.

As noted by @desrosj during the 5.4.2 release, the contrast for the update notice on the About page in WordPress 5.3.x is very poor because the background is also white. It appears to also apply to 5.4.x, as the background is the same.

Props to @desrosj for the screenshots :)

[afercia]

As long as the update notice text and links do have sufficient contrast, I wouldn't say this is a big accessibility issues. The fact the notice container "shape" isn't that visible is more a design thing as notices on white backgrounds are expected to use the notice-alt CSS class.

Not sure how the "upgrade nag" can differentiate the CSS classes depending on the screen, though.

[SergeyBiryukov]

Would it be an option to add a more visible border to the update notices on this screen?

[desrosj]

Screen Shot 2020-06-12 at 09.40.20.png​ is achieved by adding the .notice .notice-alt classes to .update-nag. As an able user, it feels weird to have a yellow line with text next to it. It looks broken in my opinion. Something like the screenshot I just uploaded at least makes it clear that notice belongs.

[afercia]

This ticket was discussed during today's accessibility bug-scrub: the team agreed this issue isn't strictly related to accessibility. Removing the accessibility focus.

[nikhilbhansi]

Border property was missing for the 'update-nag' class.

[SergeyBiryukov]

In 48316:

Administration: Bring the update nag in line with other admin notices.

In addition to more consistent display, this gives it a visible border on the About page.

Props desrosj, nikhilbhansi, afercia, SergeyBiryukov.
Fixes #50372.